current status of nanomedicine

REVIEW Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Computational and Theoretical Nanoscience Vol. 2, 1–25, 2005 Current Status of Nanomedicine and Medical Nanorobotics Robert A. Freitas, Jr. Institute for Molecular Manufacturing, Palo Alto, California, USA Nanomedicine is the process of diagnosing, treating, and preventing disease and traumatic injury, of relieving pain, and of preserving and improving human health, using molecular tools and molecular knowledge of the human body. In the relatively near term, nanomedicine can address many impor- tant medical problems by using nanoscale-structured materials and simple nanodevices that can be manufactured today, including the interaction of nanostructured materials with biological systems. In the mid-term, biotechnology will make possible even more remarkable advances in molecular medicine and biobotics, including microbiological biorobots or engineered organisms. In the longer term, perhaps 10–20 years from today, the earliest molecular machine systems and nanorobots may join the medical armamentarium, finally giving physicians the most potent tools imaginable to conquer human disease, ill-health, and aging. Keywords: Assembly, Nanomaterials, Nanorobot, Nanorobotics, Nanotechnology. CONTENTS 1. Nanotechnology and Nanomedicine . . . . . . . . . . . . . . . . . . 1 2. Medical Nanomaterials and Nanodevices . . . . . . . . . . . . . . 2 2.1. Nanopores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. Artificial Binding Sites and Molecular Imprinting . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Quantum Dots and Nanocrystals . . . . . . . . . . . . . . . . . 3 2.4. Fullerenes and Nanotubes . . . . . . . . . . . . . . . . . . . . . 4 2.5. Nanoshells and Magnetic Nanoprobes . . . . . . . . . . . . . 4 2.6. Targeted Nanoparticles and Smart Drugs . . . . . . . . . . . 5 2.7. Dendrimers and Dendrimer-Based Devices . . . . . . . . . . 6 2.8. Radio-Controlled Biomolecules . . . . . . . . . . . . . . . . . 7 3. Microscale Biological Robots . . . . . . . . . . . . . . . . . . . . . . 8 4. Medical Nanorobotics . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Early Thinking in Medical Nanorobotics . . . . . . . . . . . 9 4.2. Nanorobot Parts and Components . . . . . . . . . . . . . . . . 9 4.3. Self-Assembly and Directed Parts Assembly . . . . . . . . . 12 4.4. Positional Assembly and Molecular Manufacturing . . . . . 14 4.5. Medical Nanorobot Designs and Scaling Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 1. NANOTECHNOLOGY AND NANOMEDICINE Annual U.S. federal funding for nanotechnology R&D exceeded $500 million in 2002 1 reaching $849 million in FY 2004 2 and could approach $1 billion in next year’s budget. The European Commission has set aside 1.3 bil- lion euros for nanotechnology research during 2003– 2006, 3 with annual nanotechnology investment worldwide reaching approximately $3 billion in 2003. The world- wide market for nanoscale devices and molecular model- ing should grow 28%/year, rising from $406 million in 2002 to $1.37 billion in 2007, with a 35%/year growth rate in revenues from biomedical nanoscale devices. 4 In December 2002 the U.S. National Institutes of Health announced a 4-year program for nanoscience and nanotechnology in medicine. 3 Burgeoning interest in the medical applications of nanotechnology has led to the emergence of a new field called nanomedicine. 3 5–12 Most broadly, nanomedicine is the process of diagnosing, treat- ing, and preventing disease and traumatic injury, of reliev- ing pain, and of preserving and improving human health, using molecular tools and molecular knowledge of the human body. 5 The NIH Roadmap’s new Nanomedicine Initiatives, first released in late 2003, “envision that this cutting-edge area of research will begin yielding medical benefits as early as 10 years from now” and will begin with “establishing a handful of Nanomedicine Centersstaffed by a highly interdisciplinary scientific crew including biologists, physicians, mathematicians, J. Comput. Theor. Nanosci. 2005, Vol. 2, No. 1 1546-198X/2005/2/1/025/$17.00+.25 doi:10.1166/jctn.2005.01 1 REVIEW Current Status of Nanomedicine and Medical Nanorobotics Freitas engineers and computer scientistsgathering extensive information about how molecular machines are built” who will also develop “a new kind of vocabulary—lexicon— to define biological parts and processes in engineering terms”. 13 Even state-funded programs have begun, such as New York’s Alliance for Nanomedical Technologies. 14 In the relatively near term, over the next 5 years, nanomedicine can address many important medical prob- lems by using nanoscale-structured materials and simple nanodevices that can be manufactured today (Section 2). This includes the interaction of nanostructured materi- als with biological systems. 7 Over the next 5–10 years, biotechnology will make possible even more remark- able advances in molecular medicine and biobotics–micro- biological robots or engineered organisms (Section 3). In the longer term, perhaps 10–20 years from today, the ear- liest molecular machine systems and nanorobots may join the medical armamentarium, finally giving physicians the most potent tools imaginable to conquer human disease, ill-health, and aging (Section 4). 2. MEDICAL NANOMATERIALS AND NANODEVICES 2.1. Nanopores Perhaps one of the simplest medical nanomaterials is a sur- face perforated with holes, or nanopores. In 1997 Desai and Ferrari created what could be considered one of the ear- liest therapeutically useful nanomedical devices, 15 employ- ing bulk micromachining to fabricate tiny cell-containing chambers within single crystalline silicon wafers. The chambers interface with the surrounding biological envi- ronment through polycrystalline silicon filter membranes which are micromachined to present a high density of uni- form nanopores as small as 20 nanometers in diameter. These pores are large enough to allow small molecules such as oxygen, glucose, and insulin to pass, but are small enough to impede the passage of much larger immune system molecules such as immunoglobulins and Robert A. Freitas, Jr. is Senior Research Fellow at the Institute for Molecular Manufactur- ing (IMM) in Palo Alto, California, and was a Research Scientist at Zyvex Corp. (Richard- son, Texas), the first molecular nanotechnology company, during 2000-2004. He received B.S. degrees in Physics and Psychology from Harvey Mudd College in 1974 and a J.D. from University of Santa Clara in 1979. Freitas co-edited the 1980 NASA feasibility analysis of self-replicating space factories and in 1996 authored the first detailed technical design study of a medical nanorobot ever published in a peer-reviewed mainstream biomedical journal. More recently, Freitas is the author of Nanomedicine, the first book-length technical discussion of the potential medical applications of molecular nanotechnology and medical nanorobotics; the first two volumes of this 4-volume series were published in 1999 and 2003 by Landes Bioscience. His research interests include: nanomedicine, medical nanorobotics design, molecular machine systems, diamond mechanosynthesis (theory and experimental pathways), molecular assemblers and nanofactories, and self-replication in machine and factory systems. He has published 25 ref- ereed journal publications and several contributed book chapters, and most recently co-authored Kinematic Self-Replicating Machines (2004), another first-of-its-kind technical treatise. graft-borne virus particles. Safely ensconced behind this artificial barrier, immunoisolated encapsulated rat pancre- atic cells may receive nutrients and remain healthy for weeks, secreting insulin back out through the pores while the immune system remains unaware of the foreign cells which it would normally attack and reject. Microcap- sules containing replacement islets of Langerhans cells— most likely easily-harvested piglet islet cells—could be implanted beneath the skin of some diabetes patients. 16 This could temporarily restore the body’s delicate glu- cose control feedback loop without the need for powerful immunosuppressants that can leave the patient at serious risk for infection. Supplying encapsulated new cells to the body could also be a valuable way to treat other enzyme or hormone deficiency diseases, 17 including encapsulated neurons which could be implanted in the brain and then be electrically stimulated to release neurotransmitters, possi- bly as part of a future treatment for Alzheimer’s or Parkin- son’s diseases. The flow of materials through nanopores can also be externally regulated. 18 The first artificial voltage-gated molecular nanosieve was fabricated by Martin and collea- gues 19 in 1995. Martin’s membrane contains an array of cylindrical gold nanotubules with inside diameters as small as 1.6 nanometers. When the tubules are positively charged, positive ions are excluded and only negative ions are transported through the membrane. When the mem- brane receives a negative voltage, only positive ions can pass. Future similar nanodevices may combine voltage gat- ing with pore size, shape, and charge constraints to achieve precise control of ion transport with significant molecu- lar specificity. Martin’s recent efforts 20 have been directed at immobilizing biochemical molecular-recognition agents such as enzymes, antibodies, other proteins and DNA inside the nanotubes as active biological nanosensors, 21 to perform drug separations, 22 23 and to allow selected biocatalysis. 23 Others are investigating synthetic nanopore ion pumps, 24 voltage-gated nanopores embedded in artifi- cial membranes, 25 and an ion channel switch biosensor 26 that detects changes in chemical concentration of ∼10 −18 . 2 J. Comput. Theor. Nanosci. 2, 1–25, 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics Molecular dynamics theoretical studies of viscosity 27 and diffusion 28 through nanopores are in progress. Finally, Daniel Branton’s team at Harvard University has conducted an ongoing series of experiments using an electric field to drive a variety of RNA and DNA poly- mers through the central nanopore of an alpha-hemolysin protein channel mounted in a lipid bilayer similar to the outer membrane of a living cell. 29 By 1998, Bran- ton had shown that the nanopore could be used to rapidly discriminate between pyrimidine and purine seg- ments (the two types of nucleotide bases) along a sin- gle RNA molecule. In 2000, the scientists demonstrated the ability to distinguish between DNA chains of simi- lar length and composition that differ only in base pair sequence. Current research is directed toward reliably fabricating pores with specific diameters and repeatable geometries at high precision, 30 understanding the unzip- ping of double-stranded DNA as one strand is pulled through the pore 31 and the recognition of folded DNA molecules passing through the pore, 32 experiments with new 3–10 nm silicon-nitride nanopores, 32 and investigating the benefits of adding electrically conducting electrodes to pores to improve longitudinal resolution “possibly to the single-base level for DNA”. 32 Nanopore-based DNA- sequencing devices could allow per-pore read rates poten- tially up to 1000 bases per second, 33 possibly eventually providing a low-cost high-throughput method for very rapid genome sequencing. 2.2. Artificial Binding Sites and Molecular Imprinting Another early goal of nanomedicine is to study how bio- logical molecular receptors work, and then to build arti- ficial binding sites on a made-to-order basis to achieve specific medical results. Molecular imprinting 34 35 is an existing technique in which a cocktail of functionalized monomers interacts reversibly with a target molecule using only noncovalent forces. The complex is then cross-linked and polymerized in a casting procedure, leaving behind a polymer with recognition sites complementary to the tar- get molecule in both shape and functionality. Each such site constitutes an induced molecular “memory,” capable of selectively binding the target species. In one experi- ment involving an amino acid derivative target, one artifi- cial binding site per (3.8 nm) 3 polymer block was created. Chiral separations, enzymatic transition state activity, and high receptor affinities have been demonstrated. Molecularly imprinted polymers could be medically useful in clinical applications such as controlled drug release, drug monitoring devices, quick biochemical sep- arations and assays, 36 recognition elements in biosensors and chemosensors, 37 and biological and receptor mimics including artificial antibodies (plastibodies) or biomimick- ing enzymes (plastizymes). 37 But molecularly imprinted polymers have limitations, such as incomplete template removal, broad guest affinities and selectivities, and slow mass transfer. Imprinting inside dendrimers (Section 2.7) may allow quantitative template removal, nearly homoge- neous binding sites, solubility in common organic solvents, and amenability to the incorporation of other functional groups. 35 2.3. Quantum Dots and Nanocrystals Fluorescent tags are commonplace in medicine and biol- ogy, found in everything from HIV tests to experiments that image the inner functions of cells. But different dye molecules must be used for each color, color-matched lasers are needed to get each dye to fluoresce, and dye colors tend to bleed together and fade quickly after one use. “Quantum dot” nanocrystals have none of these short- comings. These dots are tiny particles measuring only a few nanometers across, about the same size as a protein molecule or a short sequence of DNA. They come in a nearly unlimited palette of sharply-defined colors which can be customized by changing particle size or composi- tion. Particles can be excited to fluorescence with white light, can be linked to biomolecules to form long-lived sensitive probes to identify specific compounds up to a thousand times brighter than conventional dyes used in many biological tests, and can track biological events by simultaneously tagging each biological component (e.g., different proteins or DNA sequences) with nanodots of a specific color. Quantum Dot Corp. (www.qdots.com), the manufac- turer, believes this kind of flexibility could offer a cheap and easy way to screen a blood sample for the presence of a number of different viruses at the same time. It could also give physicians a fast diagnostic tool to detect, say, the presence of a particular set of proteins that strongly indi- cates a person is having a heart attack or to detect known cellular cancer markers. 38 On the research front, the abil- ity to simultaneously tag multiple biomolecules both on and inside cells could allow scientists to watch the com- plex cellular changes and events associated with disease, providing valuable clues for the development of future pharmaceuticals and therapeutics. Quantum dots are use- ful for studying genes, proteins and drug targets in single cells, tissue specimens, and living animals. 39 Quantum dots are being investigated as chemical sensors, 40 for cancer cell detection, 38 gene expression studies, 41 gene map- ping and DNA microarray analysis, 42 immunocytochem- ical probes, 43 intracellular organelle markers, 44 live cell labeling, 45 46 medical diagnostics and drug screening, 47 SNP (Single Nucleotide Polymorphism) genotyping, 48 vas- cular imaging, 49 and many other applications. 50 51 Quan- tum dot physics has been studied theoretically 52 and computationally using time-dependent density functional theory 53 and other methods. 54–56 J. Comput. Theor. Nanosci. 2, 1–25, 2005 3 REVIEW Current Status of Nanomedicine and Medical Nanorobotics Freitas Researchers from Northwestern University and Argonne National Laboratory have created a hybrid “nanodevice” composed of 4.5-nm nanocrystals of biocompatible tita- nium dioxide semiconductor covalently attached with snip- pets of oligonucleotide DNA. 57 Experiments showed that these nanocomposites not only retain the intrinsic pho- tocatalytic capacity of TiO 2 and the bioactivity of the oligonucleotide DNA, but more importantly also pos- sess the unique property of a light-inducible nucleic acid endonuclease (separating when exposed to light or x-rays). For example, researchers would attach to the semiconduc- tor scaffolding a strand of DNA that matches a defective gene within a cell, then introduce the nanoparticle into the cell nucleus where the attached DNA binds with its defec- tive complementary DNA strand, whereupon exposure of the bound nanoparticle to light or x-rays snips off the defective gene. Other molecules besides oligonucleotides can be attached to the titanium dioxide scaffolding, such as navigational peptides or proteins, which, like viral vectors, can help the nanoparticles home in on the cell nucleus. This simple nanocrystal nanodevice might one day be used to target defective genes that play a role in cancer, neu- rological disease and other conditions, though testing in a laboratory model is at least two years away. 58 2.4. Fullerenes and Nanotubes Soluble derivatives of fullerenes such as C 60 have shown great utility as pharmaceutical agents. These derivatives, many already in clinical trials (www.csixty.com), have good biocompatibility and low toxicity even at rela- tively high dosages. Fullerene compounds may serve as antiviral agents (most notably against HIV, 59 where they have also been investigated computationally 60 61 ), antibacterial agents (E. coli, 62 Streptococcus, 63 Mycobac- terium tuberculosis, 64 etc.), photodynamic antitumor 65 66 and anticancer 67 therapies, antioxidants and anti-apoptosis agents which may include treatments for amyotrophic lat- eral sclerosis (ALS or Lou Gehrig’s disease) 68 and Parkin- son’s disease. Single-walled 69 70 and multi-walled 71–73 carbon nanotubes are being investigated as biosensors, for example to detect glucose, 72 74 ethanol, 74 hydrogen peroxide, 71 selected proteins such as immunoglobulins, 70 and as an electrochemical DNA hybridization biosensor. 69 2.5. Nanoshells and Magnetic Nanoprobes Halas and West at Rice University in Houston have devel- oped a platform for nanoscale drug delivery called the nanoshell. 75 76 Unlike carbon fullerenes, the slightly larger nanoshells are dielectric-metal nanospheres with a core of silica and a gold coating, whose optical resonance is a function of the relative size of the constituent layers. The nanoshells are embedded in a drug-containing tumor- targeted hydrogel polymer and injected into the body. The shells circulate through the body until they accumulate near tumor cells. When heated with an infrared laser, the nanoshells (each slightly larger than a polio virus) selec- tively absorb the IR frequencies, melt the polymer and release their drug payload at a specific site. Nanoshells offer advantages over traditional cancer treatments: earlier detection, more detailed imaging, fast noninvasive imag- ing, and integrated detection and treatment. 77 This tech- nique could also prove useful in treating diabetes. Instead of taking an injection of insulin, a patient would use a ballpoint-pen-size infrared laser to heat the skin where the nanoshell polymer had been injected. The heat from nanoshells would cause the polymer to release a pulse of insulin. Unlike injections, which are taken several times a day, the nanoshell-polymer system could remain in the body for months. Nanospectra Biosciences (www.nanospectra.com), a pri- vate company started by Halas and West, is develop- ing commercial applications of nanoshell technology. Nanospectra is conducting animal studies at the MD Anderson Cancer Center at the University of Texas, specif- ically targeting micrometastases, tiny aggregates of can- cer cells too small for surgeons to find and remove with a scalpel. The company hopes to start clinical tri- als for the cancer treatment by 2004 and for the insulin- delivery system by 2006. In mid-2003, Rice researchers announced the development of a point-of-care whole blood immunoassay using antibody-nanoparticle conjugates of gold nanoshells. 78 Varying the thickness of the metal shell allow precise tuning of the color of light to which the nanoshells respond; near-infrared light penetrates whole blood very well, so it is an optimal wavelength for whole blood immunoassay. 79 Successful detection of sub-nanogram-per-milliliter quantities of immunoglobu- lins was achieved in saline, serum, and whole blood in 10–30 minutes. 78 An alternative approach pursued by Triton BioSystems (www.tritonbiosystems.com) is to bond iron nanoparticles and monoclonal antibodies into nanobioprobes about 40 nanometers long. The chemically inert probes are injected and circulate inside the body, whereupon the antibodies selectively bind to tumor cell membranes. Once the tumor (whether visible or micrometastases) is covered with bio- probes after several hours, a magnetic field generated from a portable alternating magnetic field machine (similar to a miniaturized MRI machine) heats the iron particles to more than 170 degrees, killing the tumor cells in a few seconds. 80 Once the cells are destroyed, the body’s excre- tion system removes cellular residue and nanoparticles alike. Test subjects feel no pain from the heat generated. 80 Triton BioSystems plans to start designing human tests and ask the FDA for permission to begin human clinical trials in 2006. Mirkin’s group at Northwestern University uses mag- netic microparticle probes coated with target protein- binding antibodies plus 13-nm nanoparticle probes with a 4 J. Comput. Theor. Nanosci. 2, 1–25, 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics similar coating but including a unique hybridized “bar- code” DNA sequence as an ultrasensitive method for detecting protein analytes such as prostate-specific anti- gen (PSA). 81 After the target protein in the test sample is captured by the microparticles, magnetic separation of the complexed microparticle probes and PSA is followed by dehybridization of the bar-code oligonucleotides on the nanoparticle probe surface, allowing the determination of the presence of PSA by identifying the bar-code sequence released from the nanoparticle probe. Using polymerase chain reaction on the oligonucleotide bar codes allows PSA to be detected at 3 attomolar concentration, about a million times more sensitive than comparable clinically accepted conventional assays for detecting the same pro- tein target. 2.6. Targeted Nanoparticles and Smart Drugs Multisegment gold/nickel nanorods are being explored by Leong’s group at Johns Hopkins School of Medicine 82 as tissue-targeted carriers for gene delivery into cells that “can simultaneously bind compacted DNA plasmids and targeting ligands in a spatially defined manner” and allow “precise control of composition, size and multifunctional- ity of the gene-delivery system.” The nanorods are elec- trodeposited into the cylindrical 100 nm diameter pores of an alumina membrane, joining a 100 nm length gold segment and a 100 nm length nickel segment. After the alumina template is etched away, the nanorods are func- tionalized by attaching DNA plasmids to the nickel seg- ments and transferrin, a cell-targeting protein, to the gold segments, using molecular linkages that selectively bind to only one metal and thus impart biofunctionality to the nanorods in a spatially defined manner. Leong notes that extra segments could be added to the nanorods, for exam- ple to bind additional biofunctionalities such as an endo- somolytic agent, or magnetic segments could be added to allow manipulating the nanorods with an external magnetic field. Targeted radioimmunotherapeutic agents 83 include the FDA-approved “cancer smart bombs” that deliver tumor- killing radioactive yttrium (Zevalin) or iodine (Bexxar) attached to a lymphoma-targeted (anti-CD20) antibody. 84 Other antibody-linked agents are being investigated such as the alpha-emitting actinium-based “nanogenerator” molecules that use internalizing monoclonal antibodies to penetrate the cell and have been shown, in vitro, to specifically kill leukemia, lymphoma, breast, ovarian, neu- roblastoma, and prostate cancer cells at becquerel (pic- ocurie) levels, 85 with promising preliminary results against advanced ovarian cancer in mice. 86 However, drug speci- ficity is still no better than the targeting accuracy of the chosen antibody, and there is significant mistargeting, lead- ing to unwanted side effects. Enzyme-activated drugs, first developed in the 1980s and still under active investigation, 87 separate the target- ing and activation functions. For instance, an antibody- directed enzyme-triggered prodrug cancer therapy is being developed by researchers at the University of Gottingen in Germany. 88 This targeted drug molecule turns lethal only when it reaches cancer cells while remaining harmless inside healthy cells. In tests, mice previously implanted with human tumors are given an activating targeted enzyme that sticks only to human tumor cells, mostly igno- ring healthy mouse cells. Then the antitumor molecule is injected. In its activated state, this fungal-derived antibiotic molecule is a highly-strained ring of three carbon atoms that is apt to burst open, becoming a reactive molecule that wreaks havoc among the nucleic acid molecules essential for normal cell function. But the molecule is injected as a prodrug–an antibiotic lacking the strained ring and with a sugar safety-catch. Once the sugar is clipped off by the previously positioned targeted enzyme, the drug molecule rearranges itself into a three-atom ring, becoming lethally active. Notes chemist Philip Ball: 89 “The selectivity of the damage still depends on antibody’s ability to hook onto the right cells, and on the absence of other enzymes in the body that also activate the prodrug.” A further improvement in enzyme-activated drugs are “smart drugs” that become medically active only in spe- cific circumstances and in an inherently localized manner. Yoshihisa Suzuki at Kyoto University has designed a novel drug molecule that releases antibiotic only in the pres- ence of an infection. 90 Suzuki started with the common antibiotic molecule gentamicin and bound it to a hydrogel using a newly developed peptide linker. The linker can be cleaved by a proteinase enzyme manufactured by Pseu- domonas aeruginosa, a Gram-negative bacillus that causes inflammation and urinary tract infection, folliculitis, and otitis externa in humans. Tests on rats show that when the hydrogel is applied to a wound site, the antibiotic is not released if no P. aeruginosa bacteria are present. But if any bacteria of this type are present, then the proteolytic enzyme that the microbes naturally produce cleaves the linker and the gentamicin is released, killing the bacte- ria. “If the proteinase specific to each bacterium [species] can be used for the signal,” wrote Suzuki, 90 “different spectra of antibiotics could be released from the same dressing material, depending on the strain of bacterium.” In subsequent work an alternative antibiotic release sys- tem triggered by thrombin activity, which accompanies Staphylococcus aureus wound infections, was success- fully tested as a high-specificity stimulus-responsive con- trolled drug release system. 91 Other stimulus-responsive “smart” hydrogels are being studied, including a hydrogel- composite membrane co-loaded with insulin and glucose oxidase enzyme that exhibits a twofold increase in insulin release rate when immersed in glucose solution, demon- strating “chemically stimulated controlled release” and J. Comput. Theor. Nanosci. 2, 1–25, 2005 5 REVIEW Current Status of Nanomedicine and Medical Nanorobotics Freitas “the potential of such systems to function as a chemically- synthesized artificial pancreas.” 92 Nanoparticles with an even greater range of action are being developed by Raoul Kopelman’s group at the Uni- versity of Michigan. Their current goal is the development of novel molecular nanodevices for the early detection and therapy of brain cancer, using silica-coated iron oxide nanoparticles with a biocompatible polyethylene glycol coating. 93 The miniscule size of the particles—20–200 nanometers—should allow them to penetrate into areas of the brain that would otherwise be severely damaged by invasive surgery. The particles are attached to a cancer cell antibody or other tracer molecule that adheres to cancer cells, and are affixed with a nanopacket of con- trast agent that makes the particles highly visible dur- ing magnetic resonance imaging (MRI). The particles also enhance the killing effect during the subsequent laser irradiation of brain tissue, concentrating the destructive effect only on sick cells unlike traditional chemother- apy and radiation which kills cancerous cells but also destroys healthy cells. Nanoparticles allow MRI to see a few small brain tumor cells as small as 50 microns— depending on the cancer type, tumor cells can range from 5–50 microns each and may grow in locations sep- arate from the tumor site, hence are sometimes not vis- ible to surgeons. Fei Yan, a postdoc in Kopelman’s lab, is working on these nanodevices, called the Dynamic Nano-Platform (Fig. 1), now being commercialized as therapeutic “nanosomes” under license to Molecular Ther- apeutics (www.moleculartherapeutics.com). According to the company, “the nanosome platform provides the core technology with interchangeable components that provide ultimate flexibility in targeting, imaging and treatment of cancer and cardiovascular disease indications.” 2.7. Dendrimers and Dendrimer-Based Devices Dendrimers 94 represent yet another nanostructured mate- rial that may soon find its way into medical therapeutics. 95 Starburst dendrimers are tree-shaped synthetic molecules with a regular branching structure emanating outward from a core that form nanometer by nanometer, with the number of synthetic steps or “generations” dictating the exact size of the particles, typically a few nanometers in spheroidal diameter. The peripheral layer can be made to form a dense field of molecular groups that serve as hooks for attaching other useful molecules, such as DNA, which can enter cells while avoiding triggering an immune response, unlike viral vectors commonly employed today for trans- fection. Upon encountering a living cell, dendrimers of a certain size trigger a process called endocytosis in which the cell’s outermost membrane deforms into a tiny bub- ble, or vesicle. The vesicle encloses the dendrimer which is then admitted into the cell’s interior. Once inside, the DNA is released and migrates to the nucleus where it Beacon/Sensing Molecules Photodynamic Molecules Core Matrix Reactive Oxygen Species Molecular Targets Antenna Super-Molecules Magnetic/Constant Nano-particles Cloaking PEG Coat EMonson Fig. 1. This illustration of the Dynamic Nano-Platform (DNP) or “nanosome” shows proposed extensions of the technology, which may eventually incorporate magnetic and optical control and contrast elements to enable a number of functions from biological sensing to targeted photo dynamic cancer therapy. Image courtesy of Molecular Therapeutics, Inc. and illustrator Eric E. Monson, who reserve all rights. becomes part of the cell’s genome. The technique has been tested on a variety of mammalian cell types 96 and in animal models, 97 98 though clinical human trials of den- drimer gene therapy remain to be done. Glycodendrimer “nanodecoys” have also been used to trap and deactivate some strains of influenza virus particles. 99 100 The glyco- dendrimers present a surface that mimics the sialic acid groups normally found in the mammalian cell membrane, causing virus particles to adhere to the outer branches of the decoys instead of the natural cells. In July 2003, Starpharma (www.starpharma.com) was cleared by the U.S. FDA for human trials of their dendrimer-based anti- HIV microbicide. Their product has been successful in pre- venting simian-HIV. Computational simulations have also been done on some dendrimer-based nanoparticles. 101 James Baker’s group at the University of Michigan is extending this work to the synthesis of multi-component nanodevices called tecto-dendrimers built up from a number of single-molecule dendrimer components. 102–106 Tecto-dendrimers have a single core dendrimer surrounded by additional dendrimer modules of different types, each type designed to perform a function necessary to a smart therapeutic nanodevice (Fig. 2). Baker’s group has built a library of dendrimeric components from which a combina- torially large number of nanodevices can be synthesized. 106 6 J. Comput. Theor. Nanosci. 2, 1–25, 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics Fig. 2. The standard tecto-dendrimer device, which may be composed of monitoring, sensing, therapeutic, and other useful functional modu- les. 106 Image courtesy of James Baker, University of Michigan. The initial library contains components which will per- form the following tasks: (1) diseased cell recognition, (2) diagnosis of disease state, (3) drug delivery, (4) report- ing location, and (5) reporting outcome of therapy. By using this modular architecture, an array of smart ther- apeutic nanodevices can be created with little effort. For instance, once apoptosis-reporting, contrast-enhancing, and chemotherapeutic-releasing dendrimer modules are made and attached to the core dendrimer, it should be pos- sible to make large quantities of this tecto-dendrimer as a starting material. This framework structure can be cus- tomized to fight a particular cancer simply by substituting any one of many possible distinct cancer recognition or “targeting” dendrimers, creating a nanodevice customized to destroy a specific cancer type and no other, while also sparing the healthy normal cells. In three nanodevices syn- thesized using an ethylenediamine core polyamidoamine dendrimer of generation 5, with folic acid, fluorescein, and methotrexate covalently attached to the surface to provide targeting, imaging, and intracellular drug delivery capabili- ties, the “targeted delivery improved the cytotoxic response of the cells to methotrexate 100-fold over free drug.” 105 At least a half dozen cancer cell types have already been associated with at least one unique protein which target- ing dendrimers could use to identify the cell as cancerous, and as the genomic revolution progresses it is likely that proteins unique to each kind of cancer will be identified, thus allowing Baker to design a recognition dendrimer for each type of cancer. 106 The same cell-surface pro- tein recognition-targeting strategy could be applied against virus-infected cells and parasites. Molecular modeling has been used to determine optimal dendrimer surface mod- ifications for the function of tecto-dendrimer nanode- vices and to suggest surface modifications that improve targeting. 105 NASA and the National Cancer Institute have funded Baker’s lab to produce dendrimer-based nanodevices that can detect and report cellular damage due to radiation exposure in astronauts on long-term space missions. 107 By mid-2002, the lab had built a dendrimeric nano- device to detect and report the intracellular presence of caspase-3, one of the first enzymes released during cel- lular suicide or apoptosis (programmed cell death), one sign of a radiation-damaged cell. The device includes one component that identifies the dendrimer as a blood sugar so that the nanodevice is readily absorbed into a white blood cell, and a second component using fluores- cence resonance energy transfer (FRET) that employs two closely bonded molecules. Before apoptosis, the FRET system stays bound together and the white cell interior remains dark upon illumination. Once apoptosis begins and caspase-3 is released, the bond is quickly broken and the white blood cell is awash in fluorescent light. If a reti- nal scanning device measuring the level of fluorescence inside an astronaut’s body reads above a certain baseline, counteracting drugs can be taken. 2.8. Radio-Controlled Biomolecules While there are already many examples of nanocrystals attached to biological systems for biosensing purposes, the same nanoparticles are now being investigated as a means for directly controlling biological processes. Jacob- son and colleagues 108 have attached tiny radio-frequency antennas—1.4 nanometer gold nanocrystals of less than 100 atoms—to DNA. When a ∼1 GHz radio-frequency magnetic field is transmitted into the tiny antennas, alter- nating eddy currents induced in the nanocrystals produce highly localized inductive heating, causing the double- stranded DNA to separate into two strands in a matter of seconds in a fully reversible dehybridization process that leaves neighboring molecules untouched. The long-term goal is to apply the antennas to living systems and control DNA (e.g., gene expression, the abil- ity to turn genes on or off) via remote electronic switch- ing. This requires attaching gold nanoparticles to specific oligonucleotides which, when added to a sample of DNA, would bind to complementary gene sequences, blocking the activity of those genes and effectively turning them off. Applying the rf magnetic field then heats the gold par- ticles, causing their attached DNA fragments to detach, turning the genes back on. Such a tool could give phar- maceutical researchers a way to simulate the effects of potential drugs which also turn genes on and off. 109 Says Gerald Joyce: 110 “You can even start to think of differ- ential receivers–different radio receivers that respond dif- ferently to different frequencies. By dialing in the right frequency, you can turn on tags on one part of DNA but not other tags.” The gold nanocrystals can be attached to proteins as well as DNA, opening up the possibility of future radio frequency biology electronically controlling more com- plex biological processes such as enzymatic activity, pro- tein folding and biomolecular assembly. In late 2002, Jacobson announced that his team had achieved electri- cal control over proteins as well. 111 The researchers sep- arated an RNA-hydrolyzing enzyme called ribonuclease J. Comput. Theor. Nanosci. 2, 1–25, 2005 7 REVIEW Current Status of Nanomedicine and Medical Nanorobotics Freitas S into two pieces: a large protein segment made up of 104 amino acids and a small 18-amino-acid strand called the S-peptide. The RNAase enzyme is inactive unless the small strand sits in the mouth of the protein. Jacobson’s group linked gold nanoparticles to the end of S-peptide strands and used the particles as a switch to turn the enzyme on and off—in the absence of the rf field, the S-peptides adopt their usual conformation and the RNAase remains active, but with the external rf field switched on, the rapidly spinning nanoparticles prevented the S-peptide from assembling with the larger protein, inactivating the enzyme. 3. MICROSCALE BIOLOGICAL ROBOTS One convenient shortcut to nanorobotics is to engineer nat- ural nanomachine systems—microscale biological viruses and bacteria—to create new, artificial biological devices. Efforts at purely rational virus design are underway but have not yet borne much fruit. For example, Endy et al. 112 computationally simulated the growth rates of bac- teriophage T7 mutants with altered genetic element orders and found one new genome permutation that was pre- dicted to allow the phage to grow 31% faster than wild type; unfortunately, experiments failed to confirm the pre- dicted speedup. Better models are clearly needed. 113 114 Nevertheless, combinatorial experiments on wild type T7 by others 115–117 have produced new but immunologically indistinguishable T7 variants which have 12% of their genome deleted and which replicate twice as fast as wild type. 117 The Synthetic Biology Lab at MIT (syntheticbi- ology.org) is building the next generation T7, a bacterio- phage with a genome size of about 40 Kbp and 56 genes. Considerations in the redesign process include: “adding or removing restriction sites to allow for easy manipulation of various parts, reclaiming codon usage, and eliminating parts of the genome that have no apparent function.” Young and Douglas 118 have chemically modified the Cowpea chlorotic mottle virus (CCMV) viral protein cage surface to allow engineering of surface-exposed functional groups. This includes the addition of lamanin peptide 11 (a docking site for lamanin-binding protein generously expressed on the surface of many types of breast can- cer cells) to the viral coat, and the incorporation of 180 gadolinium atoms into each 28-nm viral capsid, allowing these tumor-targeting particles to serve as tumor-selective MRI contrast agents. 119 The researchers have investigated re-engineering the artificial virion to make a complete tumor-killing nanodevice, exploiting a gating mechanism that results from reversible structural transitions in the virus. 120 The natural viral gate of CCMV has been reengi- neered to allow control by redox potential; cellular inte- riors have a higher redox potential than blood, so viral capsids could be shut tight in transit but would open their redox-controlled gates after entering targeted cancer cells, releasing their payload of therapeutic agents. In prin- ciple, the four capabilities of the engineered capsids— high-sensitivity imaging, cell targeting, drug transport, and controlled delivery—represent a potentially powerful, yet minimally toxic, way to fight metastasized cancer. 119 The rational design and synthesis of chimeric viral repli- cators is already possible today, and the rational design and synthesis of completely artificial viral sequences, lead- ing to the manufacture of completely synthetic viral repli- cators, should eventually be possible. In a three-year project 121 culminating in 2002, the 7500-base polio virus was rationally manufactured “from scratch” in the labora- tory by synthesizing the known viral genetic sequence in DNA, enzymatically creating an RNA copy of the artifi- cial DNA strand, then injecting the synthetic RNA into a cell-free broth containing a mixture of proteins taken from cells, which then directed the synthesis of complete (and fully infectious) polio virion particles. 121 Engineered bacterial “biorobots” are also being pur- sued. Mushegian 122 concludes that as few as 300 highly conserved genes are all that may be required for life, constituting the minimum possible genome for a func- tional microbe. An organism containing this minimal gene set would be able to perform the dozen or so functions required for life—manufacturing cellular biomolecules, generating energy, repairing damage, transporting salts and other molecules, responding to environmental chemical cues, and replicating. Thus a minimal synthetic microbe— a basic cellular chassis—could be specified by a genome only 150,000 nucleotide bases in length. Used in medicine, these artificial biorobots could be designed to produce use- ful vitamins, hormones, enzymes or cytokines in which a patient’s body was deficient, or to selectively absorb and metabolize into harmless end products harmful substances such as poisons, toxins, or indigestible intracellular detri- tus, or even to perform useful mechanical tasks. In November 2002, J. Craig Venter, of human genome- sequencing fame, and Hamilton O. Smith, a Nobel lau- reate, announced 123 that their new company, Institute for Biological Energy Alternatives (IBEA), had received a $3 million, three-year grant from the Energy Depart- ment to create a minimalist organism, starting with the Mycoplasma genitalium microorganism. Working with a research staff of 25 people, the scientists are removing all genetic material from the organism, then synthesizing an artificial string of genetic material resembling a natu- rally occurring chromosome that they hope will contain the minimum number of M. genitalium genes needed to sustain life. The artificial chromosome will be inserted into the hollowed-out cell, which will then be tested for its abil- ity to survive and reproduce. To ensure safety, the cell will be deliberately hobbled to render it incapable of infecting people, and will be strictly confined and designed to die if it does manage to escape into the environment. In 2003, Egea Biosciences (www.egeabiosciences.com) received “the first [patent] 124 to include broad claims for 8 J. Comput. Theor. Nanosci. 2, 1–25, 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics the chemical synthesis of entire genes and networks of genes comprising a genome, the ‘operating system’ of liv- ing organisms.” Egea’s proprietary GeneWriter™ and Pro- tein Programming™ technology has: (1) produced libraries of more than 1,000,000 programmed proteins, (2) pro- duced over 200 synthetic genes and proteins, (3) pro- duced the largest gene ever chemically synthesized of over 16,000 bases, (4) engineered proteins for novel functions, (5) improved protein expression through codon optimiza- tion, and (6) developed custom genes for protein man- ufacturing in specific host cells. Egea’s software allows researchers to author new DNA sequences that the com- pany’s hardware can then manufacture to specification with a base-placement error of only ∼10 −4 , which Egea calls “word processing for DNA”. 125 The patent recites one preferred embodiment of the invention as the synthesis of “a gene of 100,000 bpfrom one thousand 100-mers. The overlap between ‘pairs’ of plus and minus oligonu- cleotides is 75 bases, leaving a 25 base pair overhang. In this method, a combinatorial approach is used where corresponding pairs of partially complementary oligonu- cleotides are hybridized in the first step. A second round of hybridization then is undertaken with appropriately com- plementary pairs of products from the first round. This process is repeated a total of 10 times, each round of hybridization reducing the number of products by half. Ligation of the products then is performed.” The result would be a strand of DNA 100,000 base pairs in length, long enough to make a very simple bacterial genome. 125 4. MEDICAL NANOROBOTICS The third major development pathway of nanomedicine— molecular nanotechnology (MNT) or nanorobotics 5 7 126 — takes as its purview the engineering of complex nano- mechanical systems for medical applications. Just as biotechnology extends the range and efficacy of treatment options available from nanomaterials, the advent of molec- ular nanotechnology will again expand enormously the effectiveness, precision and speed of future medical treat- ments while at the same time significantly reducing their risk, cost, and invasiveness. MNT will allow doctors to perform direct in vivo surgery on individual human cells. The ability to design, construct, and deploy large num- bers of microscopic medical nanorobots will make this possible. 4.1. Early Thinking in Medical Nanorobotics In his remarkably prescient 1959 talk “There’s Plenty of Room at the Bottom,” the late Nobel physicist Richard P. Feynman proposed employing machine tools to make smaller machine tools, these to be used in turn to make still smaller machine tools, and so on all the way down to the atomic level. 127 Feynman was clearly aware of the potential medical applications of the new technology he was proposing. After discussing his ideas with a col- league, Feynman offered 127 the first known proposal for a nanomedical procedure to cure heart disease: “A friend of mine (Albert R. Hibbs) suggests a very interesting possi- bility for relatively small machines. He says that, although it is a very wild idea, it would be interesting in surgery if you could swallow the surgeon. You put the mechanical surgeon inside the blood vessel and it goes into the heart and looks around. (Of course the information has to be fed out.) It finds out which valve is the faulty one and takes a little knife and slices it out. Other small machines might be permanently incorporated in the body to assist some inad- equately functioning organ.” Later in his historic lecture in 1959, Feynman urged us to consider the possibility, in connection with biological cells, “that we can manufacture an object that maneuvers at that level!” The vision behind Feynman’s remarks became a serious area of inquiry two decades later, when K. Eric Drexler, while still a graduate student at the Massachusetts Institute of Technology, published a technical paper 128 suggesting that it might be possible to construct, from biological parts, nanodevices that could inspect the cells of a living human being and carry on repairs within them. This was followed a decade later by Drexler’s seminal technical book 126 laying the foundations for molecular machine sys- tems and molecular manufacturing, and subsequently by Freitas’ technical books 5 7 on medical nanorobotics. 4.2. Nanorobot Parts and Components 4.2.1. Nanobearings and Nanogears In order to establish the feasibility of molecular manufac- turing, it is first necessary to create and to analyze pos- sible designs for nanoscale mechanical parts that could, in principle, be manufactured. Because these components cannot yet be physically built in 2004, such designs can- not be subjected to rigorous experimental testing and validation. Designers are forced instead to rely upon ab initio structural analysis and molecular dynamics simula- tions. Notes Drexler: 126 “Our ability to model molecular machines (systems and devices) of specific kinds, designed in part for ease of modeling, has far outrun our ability to make them. Design calculations and computational experi- ments enable the theoretical studies of these devices, inde- pendent of the technologies needed to implement them.” Molecular bearings are perhaps the most convenient class of components to design because their structure and operation is fairly straightforward. One of the simplest examples is Drexler’s overlap-repulsion bearing design, 126 shown with end views and exploded views in Figure 3 using both ball-and-stick and space-filling representations. This bearing has 206 atoms of carbon, silicon, oxygen and hydrogen, and is composed of a small shaft that rotates within a ring sleeve measuring 2.2 nm in diameter. The J. Comput. Theor. Nanosci. 2, 1–25, 2005 9 REVIEW Current Status of Nanomedicine and Medical Nanorobotics Freitas 1 nm (a) (b) (c) shaft sleeveexploded view axial view side view 206 atoms Fig. 3. End views and exploded views of a 206-atom overlap-repulsion bearing. 126 Image courtesy of K. Eric Drexler. © 1992, John Wiley & Sons, Inc. Used with permission. atoms of the shaft are arranged in a 6-fold symmetry, while the ring has 14-fold symmetry, a combination that pro- vides low energy barriers to shaft rotation. Figure 4 shows an exploded view of a 2808-atom strained-shell sleeve bearing designed by Drexler and Merkle 126 using molec- ular mechanics force fields to ensure that bond lengths, bond angles, van der Waals distances, and strain energies are reasonable. This 4.8-nm diameter bearing features an interlocking-groove interface which derives from a modi- fied diamond (100) surface. Ridges on the shaft interlock with ridges on the sleeve, making a very stiff structure. Attempts to bob the shaft up or down, or rock it from side to side, or displace it in any direction (except axial rota- tion, wherein displacement is extremely smooth) encounter a very strong resistance. 129 Molecular gears are another convenient component system for molecular manufacturing design-ahead. For example, Drexler and Merkle 126 designed a 3557-atom planetary gear, shown in side, end, and exploded views in Figure 5. The entire assembly has twelve moving parts and is 4.3 nm in diameter and 4.4 nm in length, with a molecular weight of 51,009.844 daltons and a molec- ular volume of 33.458 nm 3 . An animation of the com- puter simulation shows the central shaft rotating rapidly and the peripheral output shaft rotating slowly. The small planetary gears, rotating around the central shaft, are (a) (b) exploded view 1 nm 2808 atoms Fig. 4. Exploded view of a 2808-atom strained-shell sleeve bearing. 126 Image courtesy of K. Eric Drexler. © 1992, John Wiley & Sons, Inc. Used with permission. surrounded by a ring gear that holds the planets in place and ensures that all components move in proper fashion. The ring gear is a strained silicon shell with sul- fur atom termination; the sun gear is a structure related to an oxygen-terminated diamond (100) surface; the planet gears resemble multiple hexasterane structures with oxy- gen rather than CH 2 bridges between the parallel rings; and (a) planet bearing planet carrier planet gear ring gear sun gear 3,557 atoms 1 nm (b) (c) Fig. 5. End-, side-, and exploded-view of a 3557-atom planetary gear. 126 Image courtesy of K. Eric Drexler. © 1992, John Wiley & Sons, Inc. Used with permission. 10 J. Comput. Theor. Nanosci. 2, 1–25, 2005 [...]... analogs.184 The dynamics of Brownian self-assembly,185 the theory of designable self-assembling molecular machine structures,156 186 and the computational modeling of selfassembly processes are beginning to be addressed Current Status of Nanomedicine and Medical Nanorobotics REVIEW Current Status of Nanomedicine and Medical Nanorobotics the proteosome, and the DNA replication complex Many of these protein... In the first half of the 21st century, nanomedicine should eliminate virtually all common diseases of the 20th century, and virtually all medical pain and suffering as well It is a bright future that lies ahead for nanomedicine, but we shall all have to work very long and very hard to make it come to pass Current Status of Nanomedicine and Medical Nanorobotics REVIEW Current Status of Nanomedicine and... of silicon an array of cantilevered micro-pliers which could be opened and closed electrically Boggild then used an electron beam to grow a tiny carbon nanotweezer arm from the end of each cantilever, angled so that the tips Current Status of Nanomedicine and Medical Nanorobotics REVIEW Current Status of Nanomedicine and Medical Nanorobotics enabling direct positional selection of reaction sites on... basic operations should allow building up complex and atomically precise molecular structures, permitting the manufacture of a wide range of nanoscale diamond structures with atomically precise features Current Status of Nanomedicine and Medical Nanorobotics REVIEW Current Status of Nanomedicine and Medical Nanorobotics in the pattern 1, 2, 4, 8, 16, 32, 64, etc., until some manufacturing limit is reached... complete the design.” Current Status of Nanomedicine and Medical Nanorobotics Copyright 1 MM and Xerox www.imm.org nano.xerox.com Fig 6 Side views of a 6165-atom neon gas pump/motor.132 Image courtesy of K Eric Drexler © Institute for Molecular Manufacturing (www.imm.org) which allows the design and testing of large structures or complete nanomachines and the compilation of growing libraries of molecular designs... direct parts grasping,154 155 and Saitou156 gives a simple example of “sequential random bin picking” in which a process of sequential mating of a random pair of parts drawn from a parts bin which initially contains a random assortment of parts can produce the mating of a desired pair of parts Griffith157 suggests expanding the toolbox of selfassembly by including dynamic components that emulate enzymatic... Alto, CA (2003); http://www foresight.org/Conferences/MNT11/Abstracts/Huang/index.html 141 D A Leigh, J K Y Wong, F Dehez, and F Zerbetto, Nature 424, 174 (2003) Current Status of Nanomedicine and Medical Nanorobotics REVIEW Current Status of Nanomedicine and Medical Nanorobotics 173 P W K Rothemund, Proc Natl Acad Sci (USA) 97, 984 (2000) 174 J Tien, T L Breen, and G M Whitesides, J Am Chem Soc 120,... into ATP which then serves as a fuel source for the motor, and also a chemical means of switching his hybrid motors on and off reliably:136 By engineering a secondary binding site tailored to a cell’s 11 REVIEW Current Status of Nanomedicine and Medical Nanorobotics signaling cascade, he plans to use the sensory system of the living cell to control nanodevices implanted within the cell.137 Montemagno... selectivity of all these methods relies on the exponential dependence of reaction rates on the activation barrier, which is lowered for surface reactions in a precisely defined area of the surface during mechanosynthesis The principal challenge in diamond mechanosynthesis is the controlled addition of carbon atoms to the growth surface of the diamond crystal lattice The theoretical analysis of carbon... necessitating the development of energy vs orientation modeling tools The programming of engineered sequences of such conformational switches can allow the self-assembly of quite complicated mechanical structures Saitou156 158 has presented a model of self-assembling systems in which assembly instructions are written as conformational switches—local rules that specify conformational changes of a component The . 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics the chemical synthesis of entire genes and networks of genes comprising a genome, the ‘operating system’ of liv- ing organisms.”. concentration of ∼10 −18 . 2 J. Comput. Theor. Nanosci. 2, 1–25, 2005 REVIEW Freitas Current Status of Nanomedicine and Medical Nanorobotics Molecular dynamics theoretical studies of viscosity 27 and diffusion 28 through. development of a point -of- care whole blood immunoassay using antibody-nanoparticle conjugates of gold nanoshells. 78 Varying the thickness of the metal shell allow precise tuning of the color of light